Field of the Invention
The present invention relates to an image inspecting apparatus for obtaining a distance between a reference position of a sheet and a reference image for alignment formed on the sheet (for example, a trim mark) and an image forming apparatus for correcting an image forming position in accordance with the distance between the reference position and the reference image of the sheet.
Description of the Related Art
In the image forming apparatus, the image forming position of an image formed on a sheet (hereinafter, referred to as “image position”) is different from a set position or a forming interval of images in a sheet conveying direction (sub-scanning direction) becomes sometimes uneven among a plurality of sheets continuously passed. Factors for such positional deviation of the images include deviation of a rotating speed of a photoreceptor, a transfer body and the like, deviation of a sheet conveying speed, deviation of feeding-out timing of the sheet, size accuracy of the sheet, contraction of the sheet caused by fixation processing, and the like.
In order to solve the positional deviation of the images, the image forming positions on the sheet are corrected. Usually, an edge of the sheet or the like used as a reference position, and thus a distance between the reference position and the reference image is measured, and the image position is corrected on the basis of the measured distance. However, when the reference image is read, an attitude of the sheet, that is, a height position of a sheet surface can fluctuate due to deformation such as curling of the sheet, waving or the like.
FIGS. 1A and 1B are views showing a method of detecting a distance from the reference position of the sheet to the reference image. FIG. 1A illustrates an example when detection is made by an upper reader, and FIG. 1B illustrates an example when detection is made by a lower reader.
FIG. 1A illustrates a state in which a sheet S is conveyed between an upper reader 1s and a lower reader 1b. The sheet S is curled upward in the vicinity of an end portion (front end portion) on a downstream side in a sheet conveying direction, and a height position of a sheet surface fluctuates. A trim mark T being the reference image is formed on a surface of the sheet S. A reference surface Pf is a surface including positions of focal points (just focused) of optical systems provided by the reader 1s and the reader 1b, respectively. A distance (length) Ls between the front end portion and the trim mark T along the sheet surface is a distance to be detected by the upper reader 1s. However, actually, a distance Er1 between the front end portion and the trim mark T in the sheet conveying direction is erroneously detected.
Also in FIG. 1B, the sheet S is curled downward in the vicinity of the front end portion, and the height position of the sheet surface fluctuates in the same way as in the case of FIG. 1A. The trim mark T is formed on a rear surface of the sheet S. The distance (length) Lb between the front end portion and the trim mark T along the sheet surface is a distance to be detected by the lower reader 1b. However, actually, a distance Er2 between the front end portion and the trim mark T in the sheet conveying direction is erroneously detected.
When the height position of the sheet surface fluctuates as above, the distance from the reference position of the sheet to the reference image cannot be measured accurately, which causes a problem in which alignment cannot be performed with accuracy. Particularly, in a one-pass both-side detection region without backup such as a guide member or the like in a sheet conveying path, fluctuation of the height position of the sheet surface is large since the sheet surface vertically moves. Such one-pass both-side detection region as above can be found in the image forming apparatus that forms an image on, for example, both sides of the sheet.
Patent Literature 1 discloses an image forming apparatus having a reading portion that reads a sheet surface formed with a reference image and a measurement portion that measures a height position of the sheet surface at a reading position by the reading portion. The image forming apparatus disclosed in Patent Literature 1 corrects a distance between the measured reference position and the reference image of the sheet to a distance in a case where the sheet surface is flat in accordance with the height position measured by the measurement portion, and aligns the image in accordance with the corrected distance. In the image forming apparatus described in Patent Literature 1, even when the sheet is deformed, alignment can be performed by the distance between the reference position in the case where the sheet surface is flat and the reference image, and thus accurate alignment is possible.